1. Field of the Invention
The present invention relates to a code division multiple access (CDMA) receiver used for a digital portable telephone or the like, or more particularly to a synchronization tracking circuit of a CDMA receiver.
2. Description of the Related Art
A conventional CDMA receiver of this type is disclosed in JP-A-9-18446, for example.
In the conventional CDMA receiver, as shown in FIG. 1, an analog reception signal is over-sampled and converted into a digital reception signal by an A/D converter 610. A correlator 620 obtains a correlation value between the digital reception signal and a spreading code of a pilot channel (pilot spreading code) in accordance with a reception timing of the receiver. In the process, a reception timing correlator 621 obtains a correlation value between the digital reception signal and the pilot spreading code which is synchronous with a present reception timing. An early timing correlator 622 obtains a correlation value between the digital reception signal and the pilot spreading code which is synchronous with an early reception timing having a phase ahead of the present reception timing. A late timing correlator 623 obtains a correlation value between the digital reception signal and the pilot spreading code which is synchronous with a late reception timing signal having a phase lagging behind the present reception timing.
A phase difference detecting unit 630 obtains a phase difference between the reception signal and the present reception timing based on the three correlation values obtained by the correlator 620. A timing controlling unit 640 synchronizes the reception timing, which is used for obtaining the correlation values in the correlator 620, with the reception signal by shifting the reception timing in such a direction as to eliminate the phase difference obtained by the phase difference detecting unit 630. In the case where the reception timing is shifted intentionally, the direction of the phase difference to be shifted and the angle to be shifted are indicated to the timing controlling unit 640.
With the above-mentioned conventional CDMA receiver, the system configuration is restricted for selection of the sampling frequency of the A/D converter 610. Nevertheless, any sampling frequency can be selected.
An arbitrary selection of the sampling frequency, however, makes it necessary to perform a dividing operation for calculating, setting and implementing a correction value of the reception timing when the phase difference between the reception signal and the reception timing is detected to correct the reception timing. The result is longer time required for calculating the correction value of the reception timing.